The invention relates to methods and apparatus for eliminating the background residual charge of a charge image produced on a photoconductive layer. A method such as this e.g. known from the U.S. Pat. No. 4,027,158 issued May 31, 1977.
In the case of known electrophotographic procedures with photoconductive layers such as those consisting of amorphous selenium, the surface of the plate is positively or negatively charged to a high potential (approximately 1 to 5 kV). The plate prepared in this manner is exposed to the reception of irradiation such that a change in the electric conductivity is obtained in the photoconductive layer such as to affect the intensity distribution in the image. A corresponding discharge results, so that a charge image is obtained which can be rendered visible with the various means known in electrophotography. The most common procedure is e.g. to render visible by means of a coating of colored pigment which is dependent upon the charge, in particular, by means of a dusting process carried out with a dielectric powder of suitable color.
As previously already mentioned, the charge image results by virtue of the fact that the photoconductive layer is charged to a high potential in the range from about one to five kilovolts (1 to 5 kV) in relation to its base. In the exposure process, the image is then produced in that the cited charge or voltage is reduced by several hundred volts. Even an underexposed selenium plate differs from a normally exposed selenium plate only by virtue of the contrast, given an identical base optical density. The particles of the dielectric color powder--the so-called toner particles--during approximation to the charge image, are essentially orientated to the fields produced as the consequence of the charge differences and possibly as a consequence of separate auxiliary fields. The toner granules must here be selected in such a fashion that they will result in an image in spite of the high background (or base) residual charge of the plate. They cannot be overly sensitive; i.e., too finely granulated, and they can be provided only with a relatively low specific charge.
In order for only the image-related charge peak pattern to be operative (or effective) without the background (or base) residual charge during development, if possible, in accordance with the German Auslegeschrift No. 1,522,595, for example, in order to eliminate the large background (or base) residual charge as compared with the charge differences in the image regions to be coated with toner, a charge is to be applied between the photographing of the charge-image and its visualization, said charge being correspondingly opposite to the charge of the image. As a consequence, the base (or background) is to be neutralized, and the image-related charge peak pattern can be better developed. This is to proceed e.g. in a corona discharge having a high discharge voltage. In order to avoid a leveling of the non-uniform image-related charge distribution; i.e., the image contrast, which would occur because the charge carriers follow the field lines and more strongly discharge the locations having a higher potential than those of a lower potential, a charging by means of a charge source was carried out in accordance with the U.S. Pat. No. 4,027,158, said charge source being guided in a scanning fashion at a distance over the surface (or area) to be charged, said distance lying in the order of magnitude of the desired image resolution, and wherein the charge carrier source is operated in the current saturation range. However, this method requires a very precise adherence to the distance (or spacing), a fact which present difficulties due to the large image surfaces (or areas). In addition to a precise guidance of the charge carrier source, the surface of the xerographic plate--i.e., the selenium layer--must exhibit a very precise planarity in order than no electric arcings and consequent destructions of the image will occur given the small distances between the two units.